A prior optical readout method with use of magneto-optical effect, as shown in FIG. 10(a) as an example, detects intensity of and polarization direction of a reflected light from an optical spot irradiated onto a medium 6 to form a readout signal. This method has the disadvantage that as shown in FIG. 10(b), the readout signal output extends around two times radius ro of the optical spot so that two or more marks within the optical spot cannot be resolved. The recording density of the prior method is limited by the diameter (.lambda./NA) of the optical spot where .lambda. is wavelength of the readout beam and NA is a numerical aperture of a focusing lens 7. If the wavelength is 780 nm, and the numerical aperture is 0.55, for example, the diameter of the optical spot is 1.4 .mu.m, and the size of the minimum reproducible mark is a half of it, or is limited to around 0.7 .mu.m.
A prior method of reproducing smaller mark than the diameter of the optical spot were proposed in U.S. Pat. No. 5,018,119 or the like. This prior art, as shown in FIG. 11, uses a magnetooptical recording medium having three or more magnetic layers. A part of the optical spot 2 is masked to make magnetization of the magnetic layer on the light irradiation side uniform in a single direction so that it cannot be seen as signal, thereby reproducing smaller domain 1 than the optical spot 2 at a high resolution. The masking area is formed at an area of the medium that is heated to higher temperature than a masking temperature Tm by the readout light beam.
As an example, a method of recording smaller mark than the diameter of the optical spot is as follows:
(1) Intensity of a light beam to be irradiated is controlled so that only the temperature at a center of an optical spot can exceed right over a recording temperature. This can limit a recording area to a high-temperature area (minute area) of the center of the light to form smaller mark than the optical spot.
(2) The light beam is irradiated to the medium to increase the temperature of the medium to the recording temperature and at the same time, a magnetic field is modulated at a high speed to form minute domains. This method was proposed, for example, in the Japanese Patent Laid-Open No. 54-95240.
The above-mentioned prior art have the disadvantage that the signal area is limited to the part of the optical spot so that the readout output is decreased. As a result, high S/N cannot be obtained. The above-mentioned prior arts also have the disadvantage that the domain must be formed very small to record at a high density.